A centrifuge is configured in a manner that a sample to be separated is inserted into a rotor via a vessel such as a tube or a bottle and then the rotor is rotated at a high speed thereby to separate and refine the sample. The rotors differ in the usage thereof depending on the rotation speed and include various types of rotors such as an angle rotor in which a tube hole is fixed and a swing rotor in which a bucket loaded with a tube swings from a vertical state to a horizontal state in accordance with the rotation of the rotor.
In high-speed small-amount centrifuges for separating a small amount of sample quickly, mainly the angle rotors are often employed. It is disclosed by, for example, JP-A-8-103689 that a conventional angle rotor for a centrifuge is mainly formed by cutting an aluminum block or molding plastics and is configured in a manner that a plurality of tube holes for holding the tube are disposed with the interval of a constant angle with respect to a rotation shaft. The opening end face of the tube hole is formed concentrically with the rotation shaft at its conical surface.
Some of the angle rotors are each provided with a rotor cover so as not to expose rough surfaces formed when inserting the tube into the rotor to the atmosphere in order to suppress the rotation loss due to wind to the minimum. Some kinds of rotors fabricated in recent years are arranged to allow use in a state of being attached with the rotor cover or in a state of not being attached with the rotor cover. In such a rotor, when the number of tubes containing a sample are large, the rotor cover is required to be removed and attached each time the tube having been separated is replaced by the tube before the separation. Thus, actually, an operator often uses the rotor without attaching the rotor cover.
In the case of the angle rotor disclosed in JP-A-8-103689, a resonant sound or sonorant, a so-called whistling, sound may be generated at the tube hole in which the tube is not inserted. It is disclosed by, for example, JP-A-10-34019 that a through hole communicates the space within the tube hole with the outside of the rotor is provided in order to avoid such a phenomenon.